Does anyone know of a plugin, tool or software which would change the tempo without changing the pitch of an audio file?  Spent a bunch of time recording guitars, bass, drums etc, and the singer is requesting we speed the song up. I would like to expeirment with it before re-recording all of the parts.

Many audio editing programs have tempo stretching. I use Adobe Audition. There is an inexpensive program called Slow Gold. The free Audacity might also do this. However, unless it is a tiny tempo change, you may not like the results.

Try audiosnap.

I will give this a try: http://www.youtube.com/watch?v=weIZy_ibH5c